In the field of semiconductor, conventionally, the silicon substrate (Si substrate) has been widely used as substrate material. However, in recent years, the wide-gap semiconductor substrate which is superior to the silicon substrate in physical properties has been noted. The wide-gap semiconductor substrate generally has a feature that, as compared with silicon and gallium arsenide (GaAs), the crystal lattice constant thereof is smaller and the band gap thereof is larger, and is made including at least one of silicon carbide (SiC), gallium nitride (GaN), aluminum nitride (AlN), zinc oxide (ZnO), boron (B) such as boron nitride (BN) and boron phosphide (BP), carbon (C), nitrogen (N) and oxygen (O).
The wide-gap semiconductor substrate, on the one hand, has a small crystal lattice constant, that is, a strong interatomic bond, and therefore has good physical properties, and, on the other hand, has a disadvantage that it is difficult to be etched because of its strong interatomic bond. Therefore, as a method of plasma etching the semiconductor substrate, for example, a plasma etching method for silicon carbide substrate disclosed in the Japanese Unexamined Patent Application Publication No. 2008-294210 has conventionally been suggested.
In this plasma etching method, a mask forming step of forming a silicon dioxide film (SiO2 film) having a mask pattern of a predetermined shape on a surface of a silicon carbide substrate, a first etching step of plasma etching the silicon carbide substrate using a gas mixture of SF6 gas, O2 gas and Ar gas as etching gas and using the silicon dioxide film as mask, and a second etching step of plasma etching the silicon carbide substrate using a gas mixture of Ar gas and O2 gas as etching gas and using the silicon dioxide film as mask are executed in sequence. In the first etching step, the ratio of the SF6 gas to the O2 gas to the Ar gas is set to a predetermined ratio, the ambient pressure is set to be equal to or lower than 0.5 Pa, and the silicon carbide substrate is heated to a temperature between 70° C. and 100° C. In the second etching step, the ratio of the Ar gas to the O2 gas is set to a predetermined ratio, the ambient pressure is set to be equal to or lower than 0.5 Pa, and the silicon carbide substrate is heated to a temperature between 70° C. and 100° C.